Hi all, I just bought a 91 Explorer, 4X4, 4.0 V6. Got it for dirt cheap cause it is overheating, sorta. Here's the scoop. Fellow that had it thought head gasket was blown. I checked, no water in oil, no oil in water, and let it run for about 10 minutes and did not overheat. Figured can't be blown.

When I went to look at it there was straight water in coolant system, and I noticed that Fan clutch was bad. I drove it for about 3-4 miles and temp stayed normal, about 1/2 way up guage. Drove it another 5 miles to get money from ATM and still seemed fine, until I shut it down to do the paperwork. Then, about 2-3 minutes after I shut it off I could hear it boiling over at the resevoir. Started it up and checked guage, was only 2/3 way up guage, still well within the 'Normal' brackets. Hmmmmm.... figured it was due to just water in system since water boils at 212 deg. So, I bought it anyways. Hey, 125K miles and got it for $400, figured can't go wrong.

So, I drove it about 10 miles to a friends house and drained radiator and added 1 1/2 gallons antifreeze and topped off with water. Then, drove it about 25 miles to my house. About 1/2 way still was staying less than 1/2 way up guage, so turned on A/C, still never got much over 1/2 way up guage all the way home. Got home, parked it and was not boiling over, so shut it off. Well, within a minute or two it started boiling over. Started it and temp guage was still just over 1/2 way or so. What gives?

Today I got a Block Tester. Checked it quite a bit and blue liquid never turned yellow. Used an infrared thermometer to check temp at the sending unit located where the upper hose goes into the engine, was reading just under 200 deg, guage was reading just under 1/2 way. Seems about right, no?

FWIW, I do have a freeze plug, located just above LH motor mount near front of engine, that is leaking. Any relevence?

This thing has me stumped. Any ideas? I havent' replaced thermostat or fan clutch yet cause I don't think those are the problem and don't wanna put any extra money in it unless I can figure this out first.

 

Have you replaced the coolant cap? The OEM cap is about 15-16 lbs. The cap pressure increases the temperature required for the coolant to boil. A 13 lb cap increases the boiling temp to 250 degrees. If the cap is bad, the coolant will boil at a lower temerature. The coolant is exposed to the highest temperature right after shut down as the coolant isn't flowing and disapating heat thru the radiator. It will just increase in temperature and expand. If the radiator cap spring pressure is too low, then you will get boil over. Also, I would do a good cooling system flush and also replace the fan clutch since you stated it was bad. A bad fan clutch is just another problem in the issue.